Lab 9 Manual

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2502

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Mechanical Engineering

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Jan 9, 2024

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1 | P a g e MECH 2502 Lab 9 Manual MECH 2502 - Instrumentation and Measurement Techniques Instructor: Professor Marina Freire-Gormaly Laboratory 9: Sensor applications: A strain gage-based weighing scale Fall 2023

2 | P a g e MECH 2502 Lab 9 Manual By the end of this week (lecture and laboratory) students will become familiar with: • Quarter- and half-bridge Wheatstone bridges • Strain gage temperature compensation • Strain gage self/Ohmic heating Make sure to carefully study the “Background” section of this manual and complete the “ Pre-lecture Assignment ”.

3 | P a g e MECH 2502 Lab 9 Manual 1 Background In this laboratory you will explore a typical application of a strain gage sensor in a measurement system. You will construct a weighing scale using a strain gage as schematically shown in Figure 1.1. The measurement system will consist of a cantilever beam with two attached strain gages clamped to a table. Signal conditioning is achieved by means of a Wheatstone bridge and amplification prior to the signal being acquired by a data acquisition system. You will hang a series of masses on the free end of the beam to calibrate the output of the measurement system in terms of mass. Figure 1.1 Schematic diagram of the strain gage weighing scale As part of the activities of this lab, you will: • Amplify the differential output of a Wheatstone bridge using the LM324 op-amp. • Compare the sensitivity of a quarter-bridge versus a half-bridge Whetstone bridge configuration. • Examine the effect of self/Ohmic heating due to a higher bridge excitation voltage. • Observe temperature compensation when using a half-bridge configuration • Calibrate the measurement system in terms of mass Detailed information pertaining to strain gage, operational amplifiers, and the QuansarQ8-USB DAQ is provided in the background sections of laboratory 5 and 8 manuals. Review both manuals prior to conducting this laboratory. Strain gages Cantilever beam Clamp Table Calibrated masses Wheatstone bridge Amplification Quanser Q8-USB DAQ PC/LabVIEW

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4 | P a g e MECH 2502 Lab 9 Manual 1.1 Pre-lab Assignments Prior to completing this laboratory, you are required to investigate several topics and submit your findings through Moodle as the pre-lab assignment: 1. Consider a strain gage-based scale similar to Figure 1.1. Using the calibration data provided in the table below, which relates the output signal of the system versus a series on known inputs, determine the systems: a. calibration equation b. sensitivity (mV/kg) Hint: Sensitivity of a sensor is defined as the change in output of the sensor per unit change in the physical quantity being measured. For example, a sensitivity of 10 mV/kg means the system outputs a 10 mV signal for each kilogram of applied load. Mass (kg) Amplified system output (mV) 0 10 2.5 525 5 1,050 2. In a Wheatstone bridge in a quarter-bridge configuration, recall the following relationship between bridge output voltage ( V O ), bridge excitation voltage ( V Ex ), gage factor (GF), and strain (  ): 𝑉 𝑂 𝑉 𝐸𝑥 = − ?? ∙ 𝜀 4 ( 1 1 + ?? ∙ 𝜀 2 ) Assuming that the strain gage-based scale described in question 1 has a gage factor of 2.09, uses a bridge excitation voltage of 2.5 VDC, and amplification factor of 100x, determine the induced strain (  s) for each applied mass. 3. Load cells are sensors that generate an electrical signal proportional to the force/load being measured. Using online resources identify 2 different types of load cells and provide a brief explanation of how each type operates.

5 | P a g e MECH 2502 Lab 9 Manual 2 In-Lab Exercises The exercises for this laboratory are divided into two modules. In module 1, you will implement the weighing scale using a quarter-bridge Wheatstone bridge configuration. Furthermore, you will examine how slight changes in ambient temperature affects the output of the strain gage. In module 2,you will implement a half-bridge Wheatstone bridge configuration in order to compare the sensitivity of the quarter-bridge and half-bridge configurations. Furthermore, you will examine the effect of self/Ohmic heating due to a higher bridge excitation voltage. 2.1 Module 1: Quarter-Bridge Configuration The purpose of this module is to implement a weighing scale using a cantilever beam/strain gage assembly in a quarter-bridge configuration. The output of the Wheatstone bridge will be amplified using an LM324 op-amp configured as a differential amplifier. Using the supplied VI, you will calibrate the amplified output of the bridge in terms of mass. Furthermore, you will observe how slight changes in ambient temperature affects the output of the strain gage. You are required to complete the following tasks: a) construct a quarter-bridge Wheatstone bridge circuit b) construct a differential amplifier using an LM324 op-amp that amplifies the output of the bridge circuit prior to being read by the Quanser Q8-DAQ c) calibrate the output of the circuit in terms of mass using the supplied VI and a series known masses d) observe the effect of change in ambient temperature on the output of the strain gage Experimental Setup The setup for this module consists of the following components: • Quanser Q8-USB DAQ • Flexible ruler with two mounted 2-wire strain gages • Clamp • Bench-top power supply • Handheld digital multi-meter • Prototyping breadboard • Jumper wires • LM324 op-amp • Rotary potentiometer, 1 k Ω

6 | P a g e MECH 2502 Lab 9 Manual • Resistors - 120 Ω , 2 pieces - 150 Ω , 1 piece - 800 Ω , 2 pieces - 80 k Ω , 2 pieces • RCA to Alligator cable • 100 g mass, 4 pieces Procedure Follow these steps to complete module 1: 1. Follow the instructions of the laboratory instructor on how to securely clamp the cantilever beam assembly to the laboratory bench. 2. Ensure the power supply is switched off. 3. Using the supplied components, construct the circuit shown in Figure 2.1. Refer to laboratory 8 manual to determine the LM324 pin configuration.

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7 | P a g e MECH 2502 Lab 9 Manual Figure 2.1. Connection diagram for module 1 Note: The nominal resistance of the strain gage used in the experimental setup is 120  therefore three 120  resistors are required to complete the Wheatstone quarter-bridge. However, you will notice that one of the bridge completion resistors is 150  with a 1 k  potentiometer placed in parallel. The combination of the aforementioned two resistors allows null-offsetting of the Wheatstone bridge under no-load condition. Null-offsetting is required since each of the bridge completion resistors will have sufficient deviation from their nominal resistance values (up to 5%) which will cause an unbalance in the Wheatstone bridge under no-load condition. 4. Ask your laboratory instructor to examine your circuit before turning the power supply on. 5. Switch on the power supply and adjust the output voltage to ±15 VDC. 6. Wait for 5 minutes before proceeding to the next step. This allows sufficient time for the strain gage to self heat and reach steady-state condition. 7. Ensure the Q8-USB DAQ is powered and connected to the PC via the supplied USB cable.

8 | P a g e MECH 2502 Lab 9 Manual 8. Run Strain Gage Scale.vi (shown in Figure 2.2)and set the sampling rate to 10 Hz. The VI will acquire and display the amplified output of the bridge circuit. Figure 2.2. Strain Gage Scale.vi front panel 9. Ensure no load is being exerted on the cantilever beam. Conduct a null-offset by carefully adjusting the potentiometer such that the amplified bridge output is as close as possible to 0 mV. Record the amplified output of the of the bridge circuit in Table 1. 10. Carefully load the beam with a 100 g mass. Wait until the readings stabilize before recording the amplified output of the of the bridge circuit in Table 1. 11. Without removing any masses from the beam assembly , continue taking measurements by adding additional 100 g masses one at a time until you have loaded the beam assembly with all of the available masses. Record your measurements in Table 1. Mass(es) (g) Total mass (g) Amplified bridge output (mV) 0 0 100 100 100 + 100 200 100 + 100 + 100 300 100 + 100 + 100 + 100 400

9 | P a g e MECH 2502 Lab 9 Manual Table 1. Recorded amplified quarter-bridge output during loading of the beam assembly 12. Proceed to unload the beam assembly by carefully removing each of the 100 g mass one at a time. Record the amplified bridge output each time you remove a mass. Note your measurements in Table 2. Wait until the readings stabilize before recording the amplified output of the of the bridge circuit in Table 2. Mass(es) (g) Total mass (g) Amplified bridge output (mV) 100 + 100 + 100 + 100 400 100 + 100 + 100 300 100 + 100 200 100 100 0 0 Table 2. Recorded amplified quarter-bridge output during unloading of the beam assembly 13. A quarter-bridge Wheatstone bridge implemented using a 2-wire strain gage does not compensate for changes in temperature. Examine the effect of temperature on the output of the strain gage by carefully holding a heat source (e.g. cup containing warm water, or a heat gun)directly under the strain gage. The heat dissipated form the heat source will slowly warm the strain gage and cause a drift in the readings. Observe this phenomenon and make a note of the level of observed drift. 14. Stop the VI and switch off the power supply. 2.2 Module 2: Half-Bridge Configuration The purpose of this module is to implement a weighing scale using a cantilever beam/strain gage assembly in a half-bridge configuration. Simply modify the quarter-bridge configuration that was implemented in module 1 by removing the 120 Ω bridge completion resistor located in the adjacent arm to the existing strain gage and replacing it with the second strain gage mounted on the bottom of the beam assembly (see Figure 2.3). Using the supplied VI, you will calibrate the amplified output of the bridge in terms of mass and observe a higher sensitivity with the half-bridge configuration. Furthermore, you will examine the effect of self/Ohmic heating due to a higher bridge excitation voltage. You are required to complete the following tasks: a) construct a half-bridge Wheatstone bridge circuit by modifying the circuit developed in

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10 | P a g e MECH 2502 Lab 9 Manual module 1 b) calibrate the output of the circuit in terms of mass using the supplied VI and a series known masses c) determine the output of the bridge circuit using two unknown masses d) build a voltage divider to create a 2.5 VDC excitation source and observe how lower bridge excitation leads to lower self/Ohmic heating Experimental Setup The setup for this module consists of the following components: • Quanser Q8-USB DAQ • Flexible ruler with two mounted 2-wire strain gages • Clamp • Bench-top power supply • Handheld digital multi-meter • Prototyping breadboard • Jumper wires • LM324 op-amp • Rotary potentiometer, 1 k Ω • Resistors - 100 Ω , 2 pieces - 120 Ω , 2 piece - 150 Ω , 1 piece - 800 Ω , 2 pieces - 80 k Ω , 2 pieces • RCA to Alligator cable • 100 g mass, 4 pieces • Unknown masses, 2 pieces Procedure Follow these steps to complete module 2: 1. Ensure the power supply is switched off. 2. Modify the circuit developed in module 1 to create a half-bridge Wheatstone bridge circuit. As shown in Figure 2.3, remove the 120 bridge completion resistor located in the adjacent arm to the existing strain gage and replace it with the second strain gage mounted on the bottom of the beam assembly.

11 | P a g e MECH 2502 Lab 9 Manual Figure 2.3. Connection diagram for module 2 3. Switch on the power supply and adjust the output voltage to ±15 VDC. 4. Wait for 5 minutes before proceeding to the next step. As noted earlier, this allows sufficient time for the strain gage to self heat and reach steady-state condition. 5. Ensure the Q8-USB DAQ is powered and connected to the PC via the supplied USB cable. 6. Run Strain Gage Scale.vi and set the sampling rate to 10 Hz. The VI will acquire and display the amplified output of the bridge circuit. 7. Ensure no load is being exerted on the cantilever beam. Conduct a null-offset by carefully adjusting the potentiometer such that the amplified bridge output is as close as possible to 0 mV. Record the amplified output of the of the bridge circuit in Table 3. 8. Carefully load the beam with a 100 g mass. Wait until the readings stabilize before recording the amplified output of the of the bridge circuit in Table 3.

12 | P a g e MECH 2502 Lab 9 Manual 9. Without removing any masses from the beam assembly , continue taking measurements by adding additional 100 g masses one at a time until you have loaded the beam assembly with all of the available masses. Record your measurements in Table 3. Mass(es) (g) Total mass (g) Amplified bridge output (mV) 0 0 100 100 100 + 100 200 100 + 100 + 100 300 100 + 100 + 100 + 100 400 Table 3. Recorded amplified half-bridge output during loading of the beam assembly 10. Proceed to unload the beam assembly by carefully removing each of the 100 g mass one at a time. Record the amplified bridge output each time you remove a mass. Note your measurements in Table 4. Wait until the readings stabilize before recording the amplified output of the of the bridge circuit in Table 4. Mass(es) (g) Total mass (g) Amplified bridge output (mV) 100 + 100 + 100 + 100 400 100 + 100 + 100 300 100 + 100 200 100 100 0 0 Table 4. Recorded amplified half-bridge output during unloading of the beam assembly 11. Separately load each of the unknown masses onto the beam assembly and record the amplified outputs of the bridge circuit. 12. Unlike a quarter-bridge configuration, a half-bridge Wheatstone bridge configuration compensates for thermally-induced drift. Similar to module 1, examine the effect of temperature on the output of the strain gage by carefully holding a heat source (e.g.

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13 | P a g e MECH 2502 Lab 9 Manual cup containing warm water, or a heat gun) directly under the strain gage. Place your hand over the top strain gage to ensure both strain gages are equally affected by the heat source. You should observe that heat dissipated form the heat source causes minimal drift in the readings. The reason is that the heat source equally affects both strain gages and therefore changes in resistance due to changes in temperature are cancelled out. Observe this phenomenon and make a note of the level of observed drift. 13. Observe the effect of excitation voltage on self/Ohmic heating by following these steps: a. Switch off the power supply and wait for at least a minute to allow the strain gage to reach room temperature. b. Ensure no load is being exerted on the cantilever beam. c. Switch on the power supply. You should immediately notice that the output voltage of the Wheatstone bridge circuit drifts as the strain gage self-heats due to the current passing through the gage. After a few minutes the transient effect of self-heating will diminish as the strain gage reaches steady-state temperature. Record the amount of observed drift. Note: The duration of the drift depends on many variables: voltage level, size of the strain gage, the material onto which the strain gage is mounted, ambient temperature, etc. d. Switch off the power supply. e. To minimize self-heating, lower excitation voltages are recommended (e.g. less than +2.5 VDC). f. Using the provided components, implement a voltage dividing circuit in order to create a +2.5 VDC source as shown in Figure 2.4.

14 | P a g e MECH 2502 Lab 9 Manual Figure 2.4.Schematic of a voltage divider g. Power the Wheatstone bridge with the 2.5 VDC source and perform a null- offset. h. Switch off the power supply and wait for at least a minute to allow the strain gage to reach room temperature. i. Repeat step 13(c). You should observe minimal drift in the output voltage of the Wheatstone bridge circuit. Record the amount of observed drift. j. Using the handheld multi-meter, measure and record the actual output of the voltage divider constructed in step 13(f). 14. Stop the VI and switch off the power supply.

15 | P a g e MECH 2502 Lab 9 Manual 3 Post-lab Activities 1. Using a software of your choice: a. plot a 5-point calibration curve for the quarter-bridge strain gage-based weighing scale using the data collected in Table 1 b. determine the best fit line/calibration equation c. determine the sensitivity of the system (mV/g) 2. Using the data collected in Table 2, determine if there is any noticeable hysteresis in your measurement system. 3. Using a software of your choice: a. plot a 5-point calibration curve for the half-bridge strain gage-based weighing scale using the data collected in Table 3 b. determine the best fit line/calibration equation c. determine the sensitivity of the system (mV/g) 4. Using the data collected in Table 4, determine if there is any noticeable hysteresis in your measurement system. 5. Compare the sensitivities of the quarter-bridge and half-bridge configurations. 6. Comment on how much temperature drift was observed in the outputs of both the quarter-bridge and half-bridge Wheatstone bridge circuits. Under no-load conditions the voltage dividing circuit constructed in module 2, step 13(f), outputs +2.5 VDC. However, when a load (e.g. the Wheatstone bridge) is connected to a voltage dividing circuit it caused a drop in voltage which depends on the overall resistance of the load. Assuming the Whetstone bridge has an overall resistance of R W , it can be represented as a black-box in Figure 2.5. This black-box representation of an external load in DC resistive circuits is known as the Thévenin's theorem. Using the actual output of the voltage divider measured using the handheld multi- meter in module 2, determine the overall resistance of the Wheatstone bridge circuit.

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16 | P a g e MECH 2502 Lab 9 Manual Figure 2.5. Black-box representation of an external load

17 | P a g e MECH 2502 Lab 9 Manual The content included in this manual was created as a joint venture between Quanser Inc. and Professor Nima Tabatabaei. The content is jointly copyrighted by the parties. The document and its content may not be replicated, distributed, or used in any fashion by unauthorized persons. LabVIEW ™ related content may be subject to related copyright and trademark terms by National Instruments Corporation